Abuse of prescription opioid analgesics (such as oxycodone) has become a major health concern in the United States and occurs mostly in adolescents and young adults. The goal of this application is to determine in mice the behavioral and neurobiological consequences of re-exposure to oxycodone after withdrawal from an initial adolescent oxycodone self administration, and to compare these consequences to those in animals in which initial exposure occurred in adulthood. The current application will focus on adolescent oxycodone self administration-induced alterations in rewarding and locomotor effects of oxycodone, changes in the endogenous opioid systems in the mesocorticolimbic and nigrostriatal dopaminergic pathways, and how blocking of such changes affects oxycodone self-administration behavior in adulthood. We hypothesize that changes of endogenous opioid systems in the dopaminergic pathways resulting from initial opioid exposures during adolescence play an important role in the development of opioid dependence and opioid addiction upon later re-exposure. The specific aims of this application are: 1. To determine whether and how adolescent oxycodone self-administration enhances the behavioral effect of oxycodone upon subsequent re-exposure during adulthood, in terms of development of rewarding effects and to determine whether these effects are more pronounced in adolescent than in adult mice. 2. To determine whether and how adolescent oxycodone self-administration alters mRNA levels of Pdyn, Penk1, MOP-r and KOP-r, MOP-r and KOP-r receptor density in specific brain regions of the mesocorticolimbic and nigrostriatal dopaminergic pathways, and to compare the effect in adolescent mice to those in adult mice. 3. To determine whether increased striatal MOP-r levels in the mouse brain following adolescent oxycodone self-administration underlie the greater abuse potential of oxycodone, observed upon adult re-exposure. Significance: These studies will model the early phase of prescription opioid abuse in adolescence, which has been understudied. These studies should produce important insight into the effects of adolescent oxycodone self administration on neurobiological and behavioral adaptations persisting into early adulthood. The studies could therefore eventually lead to strategies to prevent or minimize the severity of early onset prescription opioid abuse, through the identification of specific molecular and neuroanatomical substrates affected by adolescent exposure.